Project Summary Itchy skin conditions affect a substantial portion of the US population at annual costs exceeding $100 billion. Most types of chronic itch are poorly managed, establishing a compelling need to develop more effective mechanisms-based treatments for these debilitating conditions. The present proposal will investigate the role of neurons located in the rostral ventromedial medulla (RVM) that express the receptor for substance P (i.e., NK-1R) in descending modulation of itch, as well as the role of NK-1R-expressing neurons in the superficial spinal and medullary dorsal horn in the ascending transmission of itch. The proposal has two overarching aims. Specific Aim 1 hypothesizes that NK-1R-expressing neurons, and specifically ON-cells in the RVM (i.e., those that fire just prior to a noxious stimulus-evoked withdrawal response), are critically involved in descending inhibition of spinal itch transmission. We will employ chemogenetic and intracranial microinjection approaches to test if activation of NK-1R-expressing RVM neurons suppresses acute pruritogen-evoked itch behavior, or manifestations of chronic itch (spontaneous and touch-evoked scratching) in mouse models of psoriasis and atopic dermatitis itch. We will also investigate NK- 1R-mediated descending inhibition of spinal itch-transmitting neurons. Finally, we will record from optogenetically identified NK-1R-expressing neurons in RVM and functionally establish if they are ON cells. Specific Aim 2 hypothesizes that NK-1R-expressing neurons in the superficial spinal/ medullary dorsal horn give rise to ascending projections to the somatosensory thalamus and parabrachial nucleus that are critically involved in transmitting itch-related signals. We will use an optogenetic approach to determine if NK-1R- expressing neurons in the spinal/medullary dorsal horn are activated by pruritogenic stimuli, and opto- and chemogenetic approaches to investigate if the activation of such neurons elicits behavioral signs of itch (and/or pain). A better understanding of central itch modulation and ascending sensory transmission has translational significance for developing novel antipruritic treatments targeting NK-1 receptors to increase descending inhibition of itch and reduce ascending itch-related signals. Page 1